Have you by chance tried upping their protein intake? They sell pellets that are 18% or even 20% that might get them going. Also, have you tried golf balls or other 'fake eggs' in the coop? I've read that can stimulate production as well. For those that have posted that chickens aren't machines, true, but for a great many people they are part of the food chain, not pets, and economics dictate they lay eggs or get eaten. It might not hurt to keep a quarter of your flock through the winter and see what happens with them in the Spring, if you do decide to process them. At least that way, you're not totally starting from scratch in the spring, and if they lay, you could get a rooster and raise chicks from your existing birds. Good luck, either way!